A targeting vector containing a human beta-actin promoter-driven selection cassette consisting of a FRT site, lacZ sequence, the first loxP site, human beta-actin promoter driving the neomycin resistance gene, SV40 polyA, FRT site and a second loxP site was inserted downstream of exon 6 of the targeted gene, and another loxP site was inserted upstream of exon 6. Flp-mediated recombination removed the lacZ and neo cassette and left exon 6 floxed. (J:155845, J:193625)